1. Field of the Invention
The present invention relates to a method of fabricating a polysilicon film, and more particularly, to a method of fabricating a polysilicon film by an excimer laser annealing process.
2. Description of the Prior Art
The progress of science and technology has led to small, effective, and portable intelligent information products becoming a part of our lives. Display devices play an important role because all intelligent information products, such as mobile phones, personal digital assistants (PDAs), or notebooks, need display devices to be a communication interface. However, the fully developed amorphous silicon thin film transistor liquid crystal display (a-Si TFT LCD) devices, which are restricted in their carrier transfer rate, have difficulty in meeting the requirements of being thin, being power saving, and having high resolution. Therefore, the a-Si TFT LCD devices are replaced by low temperature polysilicon (LTPS) thin film transistor liquid crystal display (TFT LCD) devices.
In the liquid crystal display devices, since a normal glass substrate can only work at a temperature below 600xc2x0 C., fabricating a polysilicon film directly under a high temperature will make the glass substrate twisted. Thus, in a conventional method for fabricating a polysilicon thin film transistor, an expensive quartz substrate is needed and only a small size liquid crystal display panel can be made. Recently, a method of forming a low temperature polysilicon film by crystallizing an amorphous silicon film is gradually developed. Among the methods of forming polysilicon film, the excimer laser annealing process is the major focus.
In the excimer laser annealing process, the amorphous silicon film is melted and then crystallized by absorbing the energy irradiated from the excimer laser beams. Since the rapid absorption due to the short pulse laser only affects the surface of the polysilicon film, the glass substrate can remain in a low temperature. Normally, the excimer laser comprises a XeCl laser, ArF laser, KrF laser and XeF laser, and the excimer layers of different molecules form different wavelengths. The output energy is adjusted according to the thickness of the amorphous silicon film. For example, the output energy of the excimer laser is about 200 to 400 mJ/cm2 for an amorphous silicon film of 500 angstroms.
Please refer to FIG. 1 of a schematic diagram of fabricating a polysilicon film by an excimer laser annealing process. As shown in FIG. 1, an amorphous silicon film 12, which is about 500 angstroms in thickness, is formed on a glass substrate 10. The amorphous layer 12 can be formed in different ways, such as a low pressure chemical vapor deposition (LPCVD), plasma enhanced chemical vapor deposition (PECVD), or sputtering process.
Next, the glass substrate 10 is put into a closed chamber for performing the excimer laser annealing process. The chamber has a transparent window. The excimer laser passes through the transparent window and irradiates the amorphous silicon film 12 positioned on the glass substrate 10. According to a predetermined process boundary, the excimer laser scans the area inside the process boundary step by step. The amorphous silicon film 12 inside the process boundary is heated rapidly and forms a fully melted region and a partial melted region. According to the thermal gradient made by these two regions, a crystalloid is formed laterally toward the fully melted region by using some resident solids in the interface of these two regions as nucleuses. Therefore, a polysilicon film is formed. After that, some LCD panel fabricating processes can be followed in advance by using the polysilicon film as a source or drain to form a driving circuit in a LCD device.
However, when the amorphous silicon film 12 is formed, the deposition in each region is not uniform. Please refer to FIG. 2 of a schematic diagram of a local region near an edge of the glass substrate 10 shown in FIG. 1. As shown in FIG. 2, the amorphous silicon film 12 comprises a first region 14 and a second region 16. The amorphous silicon film 12 in the first region 14, which is closer to the center, remains a predetermined thickness, such as 500 angstroms, with a tolerance about 5% to 10%. According to the fabricating process, the amorphous silicon film 12 in the second region 16, which is closer to the edge, has a tilt sidewall in which the thickness decreases from the center to the edge.
When a process boundary of the excimer laser process is set, it is important that the amorphous silicon film 12 inside the process boundary must have enough thickness. If the amorphous silicon film 12 is not thick enough, an ablation problem is made during the excimer laser annealing process. The ablated amorphous silicon film is attached to working machines, such as the windows for laser irradiation, in a way similar to a plating method. It leads to pollute the whole working machines, and affects the following processes seriously.
In the conventional excimer laser annealing process, the processing area does not comprise the whole amorphous silicon film 12 so as avoid the pollution caused by ablation. As shown in FIG. 3, the process boundary 18 is moved from the edge of the amorphous silicon film 12 toward the center of the amorphous silicon film for a safety interval L1, typically 3-5 cm, to solve the ablation problem. However, the setting of the process boundary 18 and the quantity of the safety interval L1 is determined according to operatorsxe2x80x9d own experience. As a result, it leads to some problems. For example, when the safety interval L1 shrinks for increasing the display panel size, the working machines are polluted very often. However, when the safety interval L1 increases, the display panel size is thereby affected.
It is therefore a primary objective of the claimed invention to provide a method of setting the process boundary for fabricating a polysilicon film by using an excimer laser annealing process so as to solve the aforementioned problem.
In a preferred embodiment, the claimed invention provides a method of fabricating a polysilicon film by an excimer laser annealing process. First, an amorphous silicon film is deposited on a substrate composed of glass. The amorphous silicon film comprises a first region, which is located in the center, with a first thickness and a second region, which is located in the periphery, with a slant sidewall. The thickness of the amorphous silicon film is measured so as to obtain the profile of the sidewall in the second region. According to the thickness profile of the sidewall, a pre-curser region is determined for performing an excimer laser annealing process wherein a second thickness in the boundary of the pre-curser regionis smaller than the first thickness but greater than a critical thickness of the excimer laser annealing process so as to increase area of produced polysilicon film and avoid the ablation problem. It is an advantage of the claimed invention that the method defines the process boundary according to the profile of the sidewall of the amorphous silicon film so as to avoid the ablation problem and increase area of produced polysilicon film efficiently.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings.